We propose to continue to investigate the ontogeny of appetitive conditioning and learning, frustration, extinction and, particularly, persistence and retention of persistence in the rat. Our emphasis will be on the period between about 10 to 15 days of age, a period that may be regarded as transitional. One way to characterize this period is in terms of the form of mother-pup communication. The beginning of this transitional period, Day 10, is toward the end of the "ultrasound stage," because ultrasounds emitted by the pup at this time serve as distress signals that evoke retrieval behavior in the dam. The end of this period, Day 15, is in the first few days of the "pheromone stage," in which the straying pup locates and is attracted back to the nest by a pheromone formed in the maternal caecum and excreted in the feces. Another way to characterize this transition is in terms of the instrumentality of the pup; and our hypothesis is that in the preinstrumental (ultrasound) stage persistence is not controlled by disruptive events external to the mother-pup interaction. (There is a natural, innate persistence related to uninterrupted suckling in the pup at this age.) In the instrumental (pheromone) stage, external disruptions of ongoing behavior, such as frustrative nonreinforcements and other intense stimuli, do have a persistence-building effect. (At this age the pup will detach from a dry nipple.) Our strategy in evaluating this general developmental hypothesis will be to provide more compelling evidence than we presently have for the transitional nature of the 10-15 day period in regard to learning and retention of persistence, and at the same time to use a number of experimental manipulations to examine the ontogeny of learning, frustration and extinction.